Traveling wave tubes ("TWT"), having been developed in the mid 1940's are well known in the prior art. The characteristics of the TWT, namely, wide bandwidth, high gain, moderate peak and average power have resulted in its wide use, frequently replacing previously used microwave power amplifier tubes such as the Klystron. The main disadvantages of the TWT are that its power output capability is often less than that of the Klystron, and that, for a given output power, the TWT operates at a higher voltage. However, in many applications its wide bandwidth greatly outweighs these disadvantages.
One reason for the limitation on the peak power capability is the circuit characteristics of the TWT which make it subject to "backward wave" oscillation when using very high operating voltages. Further, TWTs typically exhibit a gross gain variation versus frequency which is approximately parabolic over the operating range. Thus, the usable, saturated or peak power output comprises a small part of the total bandwidth of the TWT.
Future trends in TWT design are being dictated by system requirements toward higher frequency, greater efficiency, increased reliability, longer life, smaller size, lighter weight, higher power, and improved gain flatness at both small signal and large signal drive levels. TWTs are thus required to operate over a broad range of frequencies and produce a fairly flat radio frequency (RF) output signal over the same frequency range.
In known systems, these requirements have generally led to design tradeoffs which may result in a reduction in efficiency of between 2% to 10% for the TWT. The TWT efficiency achievable depends upon the severity of the bandwidth and the RF flatness requirements imposed on the TWT design.
Additionally, feedback paths around or within the TWT result in gain ripple which also affects the RF output flatness over the frequency range. The effect of such a feedback signal can be either regenerative or degenerative, depending upon the closed loop phase shift. Since a TWT is "electrically long", having a phase length on the order of 10,000.degree., a small change in frequency can cause the closed loop phase shift to change by 360.degree.. A ripple pattern, with several ripple periods over the operating frequency band, is typically superimposed on the gross parabolic small signal gain characteristic of the tube.
In the prior art, achieving better response with traveling wave tubes, without the use of an equalizer, generally requires widening the performance band so that only the central part of the total band is used. It reduces the effectiveness of the interaction and results in lower tube efficiency and a longer RF circuit. Furthermore, the degree of improvement in gain flatness could be limited. Widening the band will also reduce the frequency separation of the gain peaks of the ripple pattern, which may be undesirable.